The simple answer is “it depends.” If you are working with a two-part epoxy that has a working life of approximately 30 minutes or more, and you want to perform an initial setup of the dispenser and not make any changes over the working life of the epoxy, then a positive displacement syringe system would be more accurate than an air-powered dispenser. However, if you are using a standard fluid that does not change viscosity over time, then an air-powered dispensing system and a positive displacement syringe system are comparable in accuracy.

Some people have a difficult time believing this, so let’s take a closer look at how a positive displacement syringe system works. In either an air-powered system or a mechanical positive displacement system, the syringe side is identical: a fluid-filled syringe, a plastic piston inside the syringe that rests on top of the fluid and a dispensing tip attached to the bottom of the syringe. During the operation of a positive displacement system, a rod pushes against the piston inside the syringe to force out the fluid from the dispensing tip. The conventional thinking is that the rod moves down the syringe at a given distance, displacing a specific volume of fluid . . . and that is partly true. The rod does travel a certain distance, but when it gets to the preset distance, it retracts, relieving pressure on the fluid to alleviate any dripping or excess flow from the dispensing tip. To be a “true” positive displacement system, once the rod gets to its preset distance, it would remain there until all of the fluid displaced by that movement has flowed from the dispensing tip. This is virtually impossible because the fluid would flow out of the dispensing tip at an extremely slow rate, which would be impractical in real-world dispensing applications.

A second issue is the compressibility of fluid.  Many arguments have been made regarding what constitutes a compressible fluid, but the reality is that most fluids used in dispensing systems are compressible. Since the dispensing tip provides a restriction at the end of the syringe, the compressibility of the fluid is going to come into play with regards to the accuracy of the system.

The third issue is the retraction of the rod. Again, the rod retracts so that you get the achieved shot size in a reasonable period of time while eliminating excess flow or dripping from the dispensing tip, but the rod retraction is actually “cheating.” As I mentioned earlier, a true positive displacement system would advance the rod a certain distance and remain at that position until all of the fluid has been displaced.

To summarise, although each system uses different means of force (air pressure versus mechanical pressure) to push out fluid from a syringe, the concept for each is the same and their accuracy is comparable. There really is no significant difference between the two systems.